Effects of Porphyrin Substituents on Film Structure and Photoelectrochemical Properties of Porphyrin/Fullerene Composite Clusters Electrophoretically Deposited on Nanostructured SnO2 Electrodes

AbstractWe have systematically examined the substituent effects of meso‐tetraphenylporphyrins on film structures and the photoelectrochemical properties of the composite clusters of free‐base porphyrin and C60 electrophoretically deposited on nanostructured SnO2 electrodes. The photocurrent generation efficiency was found to correlate with the complexation ability of the porphyrin for C60. Basically, the incident photon‐to‐current efficiency (IPCE) value was increased with increasing relative amounts of the porphyrin versus C60 in the films. The unique molecular arrangement of the porphyrin with the simple, specific substituents (i.e., methoxy groups at the meta‐positions of the meso‐phenyl rings of tetraphenylporphyrins (3,5‐OMeTPP; TPP=tetraphenylporphyrin)) and C60 on SnO2 electrodes resulted in the largest IPCE value (ca. 60 %) among this type of photoelectrochemical device. The rapid formation of the composite clusters and microcrystals from the combination of 3,5‐OMeTPP and C60 in a mixed solvent is unique as the association is accelerated by intermolecular interactions (i.e., hydrogen‐bonding and CH–π interactions) between the methoxy groups of the porphyrins and the porphyrin/C60, in addition to the π–π interactions between the porphyrins/C60 and C60 molecules. Both the films and single crystals composed of the porphyrin and C60 exhibited remarkably high electron mobility (7×10−2 and 0.4 cm2 V−1 s−1), which is comparable to the value for highly efficient bulk heterojunction solar cells. Our experimental results have successfully demonstrated the importance of nanostructured electron‐ and hole‐transporting pathways in bulk heterojunction solar cells. Such a finding will provide basic and valuable information on the design of molecular photovoltaics at the molecular level.